Field of the Invention
The present invention relates to a reactive photo acid-generating agent and a heat-resistant photoresist composition comprising the same. In particularly, the present invention relates to the heat-resistant photoresist composition comprising the photo acid-generating agent expressed by the following formula (1), which can increase the degree of polymerization, and polyamide oligomers having acetal or its cyclized derivatives, which have an ability of that light-exposed area is dissolved in the developer and light-unexposed area is convertible to a heat-resistant polymer in the latter heating process and thus, it can be used for passivation layer, buffer coat or layer-insulating film of the multilayer printed circuit board, 
wherein 
represent 
in which X represents xe2x80x94CH2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NHCOxe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, or 
and R represents xe2x80x94CH3, xe2x80x94C2H5, xe2x80x94C3H7, xe2x80x94C4H9, xe2x80x94CF3, 
Polyimide is useful for layer-insulating films of surface protection layers, semiconductor dielectric, multi-layer chip module, and the like due to high stability against heat and chemicals.
Conventionally, complicated lithography processes such as photoresist coating, prebaking, UV exposure, developing of photoresist, etching of passivation layer and removing of photoresist are required for patterned heat-resistant insulating layer to make electrical connection with the conductor wiring between upper and lower layer or multi-layers.
Therefore, there have been intensive researches on the preparation of direct polyimide pattern as a photosensitive compound with polyimides or polyamide precursors. If the heat-resistant materials have a function of photoresist, several steps for lithographic process such as a photoresist coating and its removing become unnecessary in making via holes for wiring. Moreover, it can reduce the use of resist and chemicals. Accordingly, the use of heat-resistant photoresist can greatly simplify and make the whole process efficient. On top of that it can prevent the degraded resolution and reproducibility which can be happened during the process of etching and resist removal. Since while photoresist is generally removed out from the top of passivation layer after lithographic processes such as light-exposure, development and etching for photoresist, the heat-resistant photoresist of the present invention remains in the semiconductor permanently, it provides not only insulation, heat resistance, mechanical and low dielectric properties which are required as semiconductor materials, but also excellent photosensitivity, resolution, transparency and developing properties which are required as photosensitive materials.
Typical examples of photosensitive chemicals are polyamic acid derivatives containing side chains bonded through ether linkage, ester linkage, amide linkage or acid ammonium salt.
The first attempt for photosensitive polyimides was using polyamic acid chromate as sensitizer [Kerwin and Goldrick, Polym: Eng. Sci., 11, 426 (1971)], which was failed to be commercialized since it used an inorganic metal and the storage period was too short.
Negative photosensitive polyamides using photo-crosslinkable polyimide precursors having esters or ionic groups as pendant groups are disclosed in German Patent No. 2,437,348, and this system indicated the possibility of applying polyimides in applications of semiconductors since soluble photosensitive polyamic acid and very little amount of a metallic ion were used. However, in general the negative heat-resistant photoresists may give us lower resolution of relief pattern due to particles or cracks present on the photomask. Further, the resolution may be deteriorated by swelling of the relief pattern, since organic solvents are used for the developer.
On the other hand, U.S. Pat. No. 4,927,736 discloses positive heat-resistant photoresists prepared from aromatic hydroxy polyimides with covalently attached or solution blended naphtoquinone diazide (NQ) as a photosensitive agent. Since aromatic hydroxy polyimides themselves absorbs large amount of light, and consequently lowering the quantum efficiency, a large amount of a photosensitive agent should be used to improve the quantum efficiency. Besides, a large amount of polar residues of used photosensitive agent remains in the film and polar groups such as xe2x80x94OHs also remain in the main chain of the polymer, and thus the dielectric constant is increased and heat-resistance is decreased.
Other methods have been reported by introducing a chemical-amplifying acid-sensitive group to aromatic hydroxy polyimides [Polymers for Advanced Technology, vol. 4, 277, 287, 1992] or polyamic t-butyl ester polyimide precursors, [European Patent Publication No. 0502400A1] to improve photosensitivity and resolution. In these methods, xe2x80x94OH or xe2x80x94COOH groups are blocked by a chemical-amplifying acid-sensitive group, thereby the solubility in aqueous alkali solutions is decreased. Then the acid produced by the photo-reaction of photo acid-generating agent decomposes the acid-sensitive group to recover xe2x80x94OH or xe2x80x94COOH groups which help the polymers to be soluble in developer solutions. Even though the quantum efficiency can be improved by the treatments, in the case of using aromatic polyimides containing hydroxyl group, the xe2x80x94OH groups generated after the thermal treatment for curing may remain in the film which increase the dielectric constant of the film; and in the case of using polyamic t-butyl ester polyimide precursors, the dielectric constant is increased and the heat-resistance is decreased because too much of photo acid-generating agent should be used.
The inventors have made intensive efforts to fundamentally solve the problems of the conventional heat-resistant photoresists such as, increase of electric constant due to hydroxyl groups generated after the thermal treatment for curing, decrease of heat-resistance due to the remaining acid-generating material in the film after the development, and especially poor flatness due to the high viscosity of polymers in the preparation of multiplayer circuit board, and to improve the photosensitivity. As a result, inventors have discovered novel positive heat-resistant photoresists enables of achieving high quantum efficiency through chemical-amplifying photosensitive agent, low dielectric constant and high heat-resistance by minimizing the use of photosensitive agent and by eliminating the unreacted polar groups, and better flatness in the preparation of multiplayer circuit board.
Accordingly, an object of the present invention is to provide an aromatic bissulphonic diamide compound containing a photo acid-generating agent.
Another object of this invention is to provide a novel heat-resistant photoresist composition comprising polyamide oligomers having ester groups as pendant groups and a photo acid-generating agent in an appropriate ratio to obtain advantages in minimizing post-exposure baking process and post-exposure delay effect owing to have.
The present invention is characterized by a photo acid-generating agent expressed by the following formula (1), 
wherein 
and R are the same as defined above.
The present invention is also characterized by a positive heat-resistant photoresist composition comprising the photo acid-generating agent expressed by the following formula (1) and the polyamide oligomer having ester groups as pendant groups expressed by the following formula (2), 
wherein Ar1 is a quaternary aromatic group which is selected from the group consisting of 
Ar2 is a secondary aromatic group which is selected from the group consisting of 
R1 and R2 are independently a hydrogen atom or C1-C10 alkyl having 
wherein R1 is a C1-C6 alkyl such as ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl and cyclohexyl; and z is an integer of 1-4 (however, excluding the case when both R1 and R2 are hydrogen atoms);
R3 and R4 are independently a hydrogen atom, 
which exist at the terminal portion of the molecule added to adjust the molecular weight of the oligomer;
a degree of polymerization (m+n) is 3-50; and
a polyamide oligomer is a homopolymer or a copolymer prepared by the combination of Ar1 and Ar2.
Hereunder is given the detailed description of the present invention.
A aromatic bissulphonic diamide compound according to the present invention expressed by the above formula (1) is a photo acid-generating agent that generates acids by absorbing light with longer than 300 nm of wavelength. It can improve the quantum efficiency and its amount can be reduced because it generates two equivalents of acids for a unit molecule. Also, it helps to increase the mechanical property, since the diimide compounds formed easily through pyrolysis increases the molecular weight through imides exchange reaction with oligomers.
Accordingly, the present invention includes a heat-resistant photoresist composition expressed by the above formula (2), which contains polyamide oligomer containing ester groups as pedant groups, together with the photo acid-generating agent expressed by formula (1). The content of the reactive photo acid-generating agent expressed by formula (1) is in the range of 0.3-15wt.% to the polyamide oligomer. If too much photo acid-generating agent is used, dielectric property, mechanical property and heat resistance worsen since the molecular weight of the polymer is not sufficient, and complete holes are not formed due to the poor optical transperancy; and otherwise if too less is used, the resolution becomes poor and mechanical strength worsens, since the acid is insufficient to function as an acid generating agent. Also, the polyamide oligomer contained in the heat-resistant photoresist composition of the present invention can improve planerization in the coating process due its low molecular weight, and it can improve the heat resistance and mechanical strength since the pyrolyzed diimide reacts with polyamide acid and functions as a sensitizer.
Ester groups contained as pedant groups in polyamide oligomers expressed by formula (2) can be carboxyl groups or acid-sensitive esters having ethers or its cyclized derivatives. Also, a low-dielectric polyimide film with excellent heat resistance and mechanical property can be prepared through the reaction of the polyamide oligomer and the photo acid-generating agent. In additions, holes formed in the pretreatment is filled due to the reduced viscosity by the low molecular weight and the planarized coating is performed by the final heat treatment.
The following Scheme 1 is an example of the reaction of the photo acid-generating agent expressed by formula (1) and the polyamide oligomers containing esters as pedant groups expressed by formula (2), 
wherein Ar1, Ar2, Ar3, R, R1, R2, R3, R4, m and n are the same as defined above.
As mentioned earlier, the heat-resistant photoresist composition according to the present invention, obtained by mixing a polyamide oligomer and a reactive photo acid-generating agent, is treated by the following process of coating, light-exposing, pre-exposure heating, developing and post-exposure baking process to provide positive photosensitive material having low-dielectric property. The light-exposed area, where the heat-resistant photoresist composition of the present invention is coated, generates acids through a decomposition of the photo acid-generating agent. Acids transform esters of the polyamide oligomer to the corresponding carboxylic acid, which transform further to polyamic acid through the acid-amplification (chain reaction of the acid) and thus the light-exposed area can be removed by dissolving in a developer. On the other hand, the light-unexposed area remains insoluble as polyamide oligomer containg pendant ester group is not dissolved during the development process. Bissulpihonic diiimide compound containing a photo acid-generating group decomposes in the post-heating process. So, the photo acid-generating group is volatilized and aromatic groups are transformed into diimide monomers. Also, the ester group of the polyamide oligomer containing ester group as pendant groups is removed by overcoming the activation energy barrier. The resultant diimide compound and polyamic acid oligomer forms the corresponding polyimide polymer having much higher molecular weight through trans-imidization process. Thus, the cracking of film, which may occur with oligomers having low molecular weight, can be prevented since the mechanical strength is increased.
Namely, when a heat-resistant photoresist composition obtained by mixing a polyamide and a photo acid-generating agent is applied on the substrates and visible or UV light are exposed through a patterned photo mask, the acid-sensitive esters are transformed to carboxyl groups. Consequently, the exposed area becomes soluble to an aqueous alkaline solution such as tetramethylammonium hydroxide(TMAH). On the other hand, as the unexposed area to light has low solubility in an alkaline solution, positive pattern of polyamide can be formed. Heating of this patterned light-unexposed area converts the acid-sensitive ester groups to carboxyl groups, and further heating causes carboxyl groups to react with neighboring amide groups to form imide groups which provides high heat resistance and low dielectric property.
This polyimides have a great heat resistance as to be resistant to temperature over 520xc2x0 C. Besides, the acid-sensitive acetal and/or its cyclic derivative decomposes to form volatile chemicals having low-molecular weight (for example, ethyl vinyl ether, dihydropyran, alkyl, alkene, ether, or cyclized alkene ether derivative thereof) and nothing remains in the film. Thus, the dielectric constant becomes much lower than that of conventional photosensitive polyimides(PSPI) using naphthoquinone diazide(NQ) as a photosensitizer.
Namely, ester groups are used in the present invention to heat-resistant reaction in place of hydroxyl groups, which deteriorate low-dielectric property, heat resistance and electrical property. And, a large amount of acids is generated while photo acid-generating agent and acid-sensitive groups used for chemical amplification process are decomposed by light or acid, and can activate decomposition of acid-sensitive groups. Consequently, the quantum efficiency is improved and the degree of polymerization is increased since reactive photo acid-generating agent participates in the polyimide polymerization process. Thus deterioration of various properties including dielectric property can be minimized.
Concentration of the acid-sensitive group (xe2x80x94COOR1) contained in the polyamide oligomer expressed by formula (2) which is contained in the heat-resistant photoresist according to the present invention is the range of 1%-90%, and preferably in the range of 15%-60%. If the concentration of acid-sensitive group (xe2x80x94COOR1) is too high, the developing rate slows down, and therefore, long light-exposure time to light or a large amount of photo acid-generating agent becomes necessary. Otherwise, if the concentration of the acid-sensitive group (xe2x80x94COOR1) is too low, there may be problems of poor resolution, decreased thickness, and difficulty in controlling developing rate due to extremely rapid development.
The heat-resistant photoresist according to the present invention can be prepared in a solution state. Examples include dimethylsulfoxide, hexamethylphosphoamide, dimethylacetamide, dimethylformamide, N-methyl-2-pyrolidone, xcex3-butyrolactone, diglyme, butoxyethanol and propyleneglycolmethylether acetate (PGMEA), and a small amount of the solvent with poor solubility, such as toluene, xylene, methanol, isopropyl alcohol, may be also used. An amount of the solvent varies with a solubility of a polyamide precursor, but its kind is not limited. However, ester-family or ether-family solvent prefers to amide-family solvent. Also, combination of two or more of the solvents can be used to improve the uniformity, thickness adjustment and adhesion of the film. The heat-resistant photoresist composition is prepared in the concentration of 10-70wt %, and it can be adjusted according to the desired coating thickness.
For a film formation using the heat-resistant photoresist composition, any one of spin coating, bar coating or doctor blade method, which are commonly used in the electronics industry, may be used. Proper drying temperature for film formation is 40-150xc2x0 C.
If the drying temperature is too low, longer drying time is required; and if it is too high, the transparency reduces since the decomposition of acid-sensitive group by the pyrolysis and the formation of imide groups, which darkens color.
The proper exposure light is visible or UV with 200-500 nm wavelength, and it is more proper to use a light exposing device equipped with a monochrome filter in order to get better resolution and processibility. In the present invention, any particular equipment or light-exposure device is not specified.
Light exposure time can be varied with experiment conditions. In the present invention, when a UV exposure device equipped with a 365 nm-filter was used, the light exposure time could be varied from 10 sec to 200 sec. If a stronger light exposure device is used, the light exposure time can be shortened. The energy of the exposure light is determined with an energy meter. The resolution is determined in depth and width with a profilometer, and the cross section of the film is identified with a scanning electronic microscope.
Polyamide oligomer used in the present invention is a well-known compound easily prepared by known methods. It can be prepared from an aromatic diamine and an aromatic dicarboxylic acid or its derivative as in the following Scheme 2, 
wherein Ar1, Ar2, R1, R2, R3, R4, m and n are the same as defined above.
In the above Scheme 2, the polymerization temperature is recommended to be under 50xc2x0 C., and preferably under 20xc2x0 C. If the polymerization temperature is too high, solvent-insoluble polyimide may form due to the excessive reaction. Also, various acid-sensitive groups (xe2x80x94COOR1) expressed in Scheme 2 may be included in the monopolymer or copolymer in order to control the development rate and the photosensitivity. For a method of bonding the acid-sensitive ester group to the polymer, it is recommendable to use excess amount of alkyl vinyl ether or cyclized alkene ether derivative in the presence of an acidic catalyst. Acid catalyst used is a strong acid such as p-toluenesulfonic acid, phosphoric acid and hydrochloric acid. The introduction of acid-sensitive agents is preferred to perform at room temperature or below. If the reaction temperature is high, the pyrolysis of acid-sensitive group and formation of polyimide may occur.